1. Field of the Invention
The present invention relates generally to a coaxial resonator used as a filter in a microwave band and a dielectric filter constructed using the coaxial resonator.
2. Description of the Prior Art
As microwave remote communication has been put into practice, there has been a demand for communication equipment that is more lightweight and smaller in size; thereby, leading to a demand to miniaturize a duplexer which is an essential component in such an equipment.
Examples of a filter used for a duplexer in a microwave band generally includes a so-called coaxial resonator constituted by an outer conductor provided on an outer peripheral surface of a dielectric member having a through hole provided therein and an inner conductor provided on an inner peripheral surface of the through hole, and a strip-line type resonator using a strip line. As to the filters, a quarter-wavelength type filter which resonates at one-fourth of a wavelength at a resonance frequency f by producing the filter in one side short-circuited construction has been generally well known. In addition, a coaxial dielectric resonator constructed using a material having a high dielectric constant (.di-elect cons.r=40 to 90) has been widely used as one suitable for miniaturization.
Particularly in a portable telephone, a receiving band and a transmission band are used proximate to each other. Accordingly, a so-called polarized method having an attenuation pole is adopted for a filter for a duplexer. Examples of such a polarized method include a method of obtaining an anti-resonance frequency by connecting a capacitor or an inductor in series with a coaxial dielectric resonator (see Japanese Utility Model Laid-Open Gazette No. 4566/1987) and a method of coupling resonators which are not adjacent to each other (see Japanese Patent Laid-Open Gazette No. 108801/1988).
FIGS. 24A and 24B illustrate a polarized filter explaining the former method, where FIG. 24A is a perspective view showing the filter, and FIG. 24B is a diagram showing an equivalent circuit thereof. Three coaxial resonators are used to fabricate a polarized band-pass filter. In this polarized band-pass filter, three coaxial resonators 62 are provided on a dielectric substrate 61 (see, FIG. 24A). Anti-resonance capacitances 63a, 63b and 63c (see FIG. 24B) are respectively connected to the coaxial resonators 62. In addition, chip capacitors 64a to 64d are provided on the dielectric substrate 61. The two chip capacitors 64b and 64c out of the chip capacitors 64a to 64d are inter-stage coupling capacitors for coupling the coaxial resonators to each other.
However, in the above described filter shown in FIG. 24A and 24B, an inter-stage coupling portion is required. Accordingly, the dielectric substrate 61 must be made relatively large so as to form the inter-stage coupling portion, thereby making it impossible to miniaturize the filter. On the other hand, in the above described method of coupling the resonators which are not adjacent to each other, a jump coupling substrate is required, thereby similarly making it impossible to miniaturize the filter.